As is known, a dichroic antenna structure consists of a surface presenting transmissive properties for radiation at certain frequencies and polarization and reflective properties for other radiation.
The most-widely used shapes for such surfaces are the round paraboloid, the round or doubly-curved ellipsoid, etc.
Generally the shape selected is the one best suited to form a reflector or subreflector of a microwave antenna system.
This surface is made of a dielectric material with good mechanical properties and on which a conductive grid, consisting of a plurality of selective antenna elements, is deposited.
This grid is generally built up by using elements having the shape of a ring, a cross, a strip, etc. The dimensions of the elements generally depend on the wavelength of the operative electromagnetic radiation and the required tolerances, both in dimensions and in the distance between the elements (interaxis or interspaced), are equal; to a few thousandths of wavelength of the reflected electromagnetic field. For instance, at a frequency of 20 GHz, the accuracy demanded is + or -100 .mu.m. This small tolerance gives rise to difficulties in applying with the desired accuracy, the great number of necessary antenna elements, taking into account, in addition, that they are to be arranged on a curved surface.
The antenna elements have generally been fabricated by a method used for manufacturing printed circuits, using photographic masking and chemical etching. However, the use of this method is not without difficulties, since the mask can only be obtained on a plane surface, while the support onto which it is to be projected can have a curved surface of the previously mentioned type.
A number of solutions to this problem are known in the art. For example, see the paper entitled Design of a Dichroic Cassegrain Subreflector by Vishwani D. Agrawal et alii, IEEE Transactions on Antennas and Propogation, Vol. AP27, No. 4, July 1979. The dichroic surface, consisting of crossed dipoles, is fabricated according to a photoetching technique on a plane Kevlar surface. This is afterwards laid on an aluminum mold of desired shape, is covered with layers of protective material and is pressed against the mold with a vacuum bag. The layer stack is finally baked in a high-temperature oven.
The accuracy attainable by this method is not very high, since the deformations undergone by the plane dichroic surface, when pressed against the curved mold, cannot be controlled. In fact these deformations depend on many factors difficult to control, such as the uniformity of the thickness of the dichroic surface, the uniformity of the pressure exerted, etc.
Another solution is described in U.S. Pat. No. 4,001,836. According to this proposal, the dichroic structure is directly fabricated on the final curved structure making a mask by applying bent strips, arranged side by side. Clearly, this method is not highly satisfactory both because strips must be manually deposited and because it is time consuming. Moreover the approach is valid only for parabolic surfaces.